The present invention has utility in applications requiring the conversion of an analog signal into a digital signal, for example for computer sensing of analog information in an automotive control system. To further illustrate, in an automotive engine control system, a microcomputer requires analog signal information from various transducers to be converted into digital signal information before it can be processed by the microcomputer. Examples of such analog signal information are the outputs of sensors for manifold pressure, oxygen, rotational speed, operator input, battery voltage, anti-knock, etc.
In a typical automotive application, many different analog signals need to be converted. The analog signals originate from many different sources and typically have different source impedances.
Most known A/D converter systems have a fixed input sample time which is determined by the hardware design of the system. The sample time should be some multiple (such as 10) of the "RC" time constant to ensure sampling to the desired accuracy. In an unbuffered charge distribution A/D converter system, a fixed sample time places restrictions on either the conversion time or the source impedance.
In prior art A/D converter systems, high impedance sources often require amplifying or long sample times, which could be adversely affected by rapidly changing signals. Long sample times also restrict the number of samples per second that can be converted. Short sample times require low source impedances, which constrain the driving circuitry.
A known A/D converter system (TI TMS370 available from Texas Instruments Corp.) utilizes software control bits by which processor software initiates sampling and then the software initiates conversion a measured time later. This is unduly burdensome on the host system software.
Thus there is a significant need to provide an A/D converter system with a variable sample time in which the host system software is freed from the responsibility of initiating the sampling operation and later the conversion operation.